Congestive heart failure (CHF) is a pathological condition characterized by avid sodium retention and edema formation. The exact mechanism underlying the exaggerated sodium reabsorption in CHF has not been clarified. Several mechanisms have been implicated including the renin angiotensin aldosterone system, sympathetic nervous system and attenuated response to atrial natriuretic peptide (ANP), an important hormone in the regulation of sodium and water homeostasis. Neutral endopeptidase (NEP), is a widely distributed enzyme, found predominantly in the kidneys and lungs, and involved in the degradation of ANP and other related peptides. We investigated 1) The renal effects of NEP inhibitor (NEP-I) before and after the induction of CHF by creation of an aortocaval fistula (ACF) in rats; 2) The expression, concentration and activity of NEP, a key enzyme in the degradation of ANP, in lungs and kidneys of rats with compensated and decomposed CHF. In control rats, NEP-I (40 mg/kg, bolus, i.v.) caused increases in urine output (UV), and both absolute (UNaV) and fractional (FENa) sodium excretion. Infusion of the same dose of NEP-I into rats with compensated CHF induced remarkable natriuresis and diuresis, whereas in rats with decompensated CHF the renal responses were markedly blunted. Quantitative reverse transcription-polymerase chain reaction (RT-PCR, 21 cycles) and Western blot analysis using polyclonal antibodies against NEP-revealed that NEP-mRNA and immunoreactivity levels were significantly lower in lungs of decompensated rats than in those of compensated and control rats. These results were further supported by visualizing NEP activity using SDS polyacrylamide gel co-polymerized with gelatin (activity gel). These findings indicate that: 1) NEP-I may be a fruitful therapeutic agent in mild, but not severe cases of CHF. 2) Renal NEP activity is not affected by severe heart failure, whereas pulmonary NEP activity is reduced by severe CHF, most likely as a compensatory adaptation to preserve ANP. 3) The blunted response to ANP in CHF is not due to changes in endogenous renal NEP.